Cilic had thrown away a break in the decider to lose against Alexander Zverev on Sunday and he managed the same again at London's O2 Arena as Sock fought back to win 5-7 6-2 7-6 (7/4).

Sock said on court afterwards he was woken up at 4am on Tuesday morning by a fire alarm at his hotel but he showed little sign of fatigue and now has his debut victory in this end-of-season tournament.

"That was a tough one for sure," Sock said. "It's been an interesting morning, a fire alarm went off at 4am and we all had to exit our hotel so it was a long morning.

"My coach growing up nicknamed me 'showtime' when I was young because I loved playing in front of people.

"There's always a lot going on in my game. Even I don't know what I'm going to do sometimes but it keeps it fun and I'm looking forward to the next one."

Jack Sock smiling during his match against Marin Cilic during day three of the NITTO ATP World Tour Finals. Photo: John Walton/PA Wire

He may still have to beat Zverev in his final group match to qualify, with the talented German going up against favourite Roger Federer later on Tuesday.

Cilic, meanwhile, plays his last round-robin match against Federer, to whom he lost in the Wimbledon final earlier this year.

The Croat was the chief aggressor in the opening frame and - after saving a Sock break point at 5-5 - he converted his own to clinch the set. Sock whacked the ball away in frustration.

Sock, however, grew more assertive, attacking Cilic's second serve, and he broke twice for a 4-1 lead then served out to level up.

But the 25-year-old's momentum failed to carry through to the decider, where Cilic stormed into a 3-0 lead.

Again, however, Sock came back, converting the second of two break points to pull even at 3-3.

Serve dominated the rest of the set but Cilic edged 4-2 up in the tie-break when the ball struck the net chord and Sock failed to dig it over.

Again Sock battled back, however. Another Cilic shot clipped the tape but this time his opponent was alive to it and flicked it down the line.

Sock claimed five points in a row to set up two match points and he converted the first when a Cilic backhand floated long.